ART: Fix underflow in codegen

Check for count == 0 first before accessing non-existent stack map.

Test: m ART_TEST_JIT=true test-art-host
Test: m ART_TEST_JIT=true test-art-host-run-test-913-heaps
Change-Id: Id4cad8e791d731147860b8a9a0d90cc893cc6972
1 file changed